1. Field of the Invention
The present invention relates to processes for producing catalyst particles. In particular, the invention relates to processes for forming catalyst particles, which processes afford excellent control of the morphology of the catalyst particles.
2. Discussion of Background Information
Carbon black is a particulate form of carbon commonly produced by thermal cracking or decomposition of a hydrocarbon raw material. More than 90 percent of carbon black is produced using an oil furnace process, which involves injecting an aromatic petroleum distillate into a preheated, closed furnace, and then cooling and collecting the carbon aggregates that have been formed. By controlled manipulation of reactor conditions, it is possible to vary particle size, aggregate size, and surface chemistry to create a broad range of carbon black properties.
Carbon black may be used, for example, in water-based systems to provide pigmentation, UV resistance, and conductivity. Carbon blacks have also found wide application as the support phase of catalyst particles, such as catalyst particles used in fuel cell membrane electrode assemblies (MEAs). In this application, a catalytically active phase is disposed on the carbon black particles. The morphology (i.e., the structure, form, or shape) and surface area characteristics of the carbon black support phase are characteristics that have a critical impact on the performance of the catalyst particles. The morphology of the catalyst that develops as the catalyst particles are formed has a strong influence, for example, on the packing density of supported catalyst particles. The packing density, in turn, impacts transportability of the reactants and products to and from, respectively, the active phase of the catalyst particles in an MEA. Generally, the greater the packing density, the more difficult it is for reactants and products to move through a fuel cell MEA. The surface area determines the type and number of surface adsorption centers where the catalytically active phase is formed. Catalyst morphology is difficult to control using known methods for catalyst synthesis. Such lack of control of catalyst morphology presents a major obstacle for the development of energy storage and production devices that contain such catalysts. Thus, the need exists for processes for forming carbon black supported catalyst particles where the morphology and surface area of the catalyst particles can be predicted and controlled.